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[Maximum Spin]

Does a de Sitter universe have no space-time curvature then? Or has localised spatial curvatures? Or is violating GR?

A de Sitter universe per se does in fact have no spacetime curvature, since it contains no ordinary mass-energy. (This is actually the whole point.)No, it turns out I was wrong about this, it's curved, but in both time and space.

An "approaching-de-Sitter-ness universe" like some models of our own universe's development predicts, of course, have negligible local spatial curvature.

[Il Palazzo]

A de Sitter universe per se does in fact have no spacetime curvature, since it contains no ordinary mass-energy. (This is actually the whole point.)

I think that's wrong. Doesn't de Sitter space-time have constant positive curvature?

[Maximum Spin]

A de Sitter universe per se does in fact have no spacetime curvature, since it contains no ordinary mass-energy. (This is actually the whole point.)

I think that's wrong. Doesn't de Sitter space-time have constant positive curvature?

No, its omega is one, so it's flat.(No, its omega is not one and physicists are filthy liars.) Its omega-lambda is also one. It does have a positive cosmological constant.

[Il Palazzo]

A de Sitter universe per se does in fact have no spacetime curvature, since it contains no ordinary mass-energy. (This is actually the whole point.)

I think that's wrong. Doesn't de Sitter space-time have constant positive curvature?

No, its omega is one, so it's flat. Its omega-lambda is also one. It does have a positive cosmological constant.

You're talking spatial curvature here.

[Maximum Spin]

You know what, never mind, I feel like this is just going to go around like this.

[Il Palazzo]

I mean, it's easy to find references discussing de Sitter space-time as being characterised by curvature. And yet, here you are, saying it isn't. I think. As you say, we might be talking past one another.

[Maximum Spin]

I mean, it's easy to find references discussing de Sitter space-time as being characterised by curvature. And yet, here you are, saying it isn't. I think. As you say, we might be talking past one another.

Having a positive cosmological constant does give a de Sitter space positive scalar curvature, which makes sense because that's basically the inverse of gravity. Basically this means that it does have global spacetime curvature and I shouldn't have said "has no spacetime curvature" before. I wasn't really considering this part but it was clearly, overall, wrong. It does appear to still be locally flat though.

However, more importantly, it also does have spatial curvature, in the sense that it is not Euclidean. This has to be the case because its curvature is reference-frame-invariant: as you move between reference frames, the space and time coordinates mix, so there definitely has to be some frame in which that curvature exists in space. I'm really confident in this and my interlocutor agrees. Apparently, it has been said that de Sitter spaces model a "flat universe", but this is a different kind of flatness, the topological kind, where all kinds of surfaces of constant positive curvature can be "flat" as long as they can be continuously deformed into the plane.

BTW, if you're thinking, "hold on, isn't 'topologically flat' just a mathematical way of saying 'not flat in any way that counts'", yes, that's what I said too. Like oh, okay, it doesn't fold in on itself like a sphere, that's not helping anybody, it could literally be hyperbolic and still meet that criterion.

[Il Palazzo]

That's fair.

[Madman198237]

So this is a thing. I was so displeased when I read further into the article and discovered that, no, it does not superconduct at room conditions, just room temperature. At 267 billion pascals of pressure.

I don't think is this much closer to being useful than the typical ones that superconduct at liquid nitrogen temperatures.

[Egan_BW]

Ya just need wire casings which squeeze the stuff real good and ya got room temp superconductors right there.

[Greiger]

That sounds like it would be fun when your cat chews through your super high pressure superconductor Ethernet cable, and the house explodes.

[Starver]

At least you aren't left wondering whether (and where) there might be a bit of cable damage. And maybe the cat won't do it again...

[bloop_bleep]

267 billion pascals? Did they drop an atom bomb on it or something? How do you even test that?

[MetalSlimeHunt]

Behold the beast.

[bloop_bleep]

I've been thinking about making, designing, or at least theorycrafting a heat pump design of some sort. Suppose I can buy stuff off of Amazon at a $30 budget (flexible if need be, for the purposes of merely designing), I can use household items already available, and I have use of a 3D printer which can print PLA plastic. For a heat pump cycle I need a compressor, then a condenser or radiator, then an expansion valve, then the "load" portion (suppose this is just a metal plate that we want to cool.) For compressor I'm wondering whether I should buy a premade one or design my own. For my own design I was thinking of a bicycle pump-like piston driven by a crank, powered by a DC motor (probably geared down). I was thinking of making this whole thing into sort of a pad form, and I was thinking of making the radiator and load portion similar, in that they are just plastic tubing looped around a whole bunch of times to maximize surface area. The question is where to put them, though? If I'm making a pad, it might make sense to put them on the top and bottom, with a layer of cotton or wool or some other insulator between them, and the condenser and expansion valve connecting them. But then the radiator would be radiating into the surface it's sitting on, which probably isn't that good as that'll likely be a wooden table or something like that. So then maybe I can put the radiator along the sides, and the middle would be freed up for more space for the compressor and expansion valve. But the radiator and the cooling tubing would be close to each other at the edges and I'm not sure if I can insulate them well enough. Then there's the question of the refrigerant to use... the wikipedia article is a bit vague on the desirable properties for one of those. Maybe I'll figure out some good materials values from the fluid mechanics formulas and then try to find a cheap, safe refrigerant that matches those, or change the parameters to match. Also, the wikipedia article mentions a fan. I guess I'd probably need a fan to keep the radiator working once it's heated up the immediately surrounding air, but I was wondering whether a fan would be strictly necessary. I could use some advice, this is an interesting thought experiment for me. Even if I don't end up building it, I'd like designing it.